As recording materials capable of recording and erasing information reversibly, photochromic materials which form or eliminate colors by irradiation with light such as spiropyran compounds have hitherto been widely studied. These materials had, however, problems in stability to light or heat, and durability in repeated use.
By contrast, as materials with light resistance that can be used repeatedly, for example, Japanese Laid-open Patent Application Sho. 54-119377 discloses organic crystal particles dispersed in a matrix polymer, in which the recording material is changed in phase by heat to form transparent states and opaque states, thereby recording and displaying the information reversibly.
This reversible thermal recording material obtained by dispersing organic crystal particles in a matrix polymer records and erases as it forms transparent states and opaque states by heating and cooling processes.
To manufacture such reversible thermal recording materials obtained by dispersing organic crystal particles in a matrix polymer, the method of applying a paint, prepared by dissolving a matrix polymer and organic crystal particles in an organic solvent, on a substrate and forming particles by a drying process has been widely employed because of the high contrast of transparent/opaque phase thereby obtained.
In particular, for dissolving both a matrix polymer and organic crystal particles and forming a recording layer capable of giving recording characteristics of high contrast, the choice of organic solvents is extremely limited, and tetrahydrofuran has been widely employed as the solvent for these purposes.
In the reversible thermal recording medium known previously in this field, generally, when the film thickness of the reversible thermal recording layer is thin, sufficient turbidity in the opaque state is not obtained, and the visibility (contrast) is inferior. To solve these problems, the film thickness of the reversible thermal recording layer must be increased; and as the film thickness increases, it is necessary to transmit heat also throughout the film's thickness, thereby requiring a large heat source.
In addition, the increases in the film thickness result in slowing the recording speed, and also require a control in heating to provide a uniform temperature throughout the thickness of the recording layer.
Besides, in a case that a reversible thermal recording material is manufactured by applying a paint prepared by dissolving a matrix polymer and organic crystal particles in tetrahydrofuran, a continuous application by an ordinary coater is difficult, and a large-scale exhaust system is needed. This is because the volatility of tetrahydrofuran is high, resulting in poor paint stability and the spread of a strong smell.
Accordingly, a search has been made for a paint using general-purpose organic solvents which have low volatility and little smell or for a water-based paint. A paint capable of completely dissolving a matrix polymer or organic crystal particles by using such general-purpose organic solvents or water has been hardly discovered, and a reversible thermal recording material of high contrast could not be obtained from such paint.
Thus, in the paint using general-purpose organic solvents or water, a matrix polymer or organic crystal particles are contained in an amount greater than its solubility, and a part of the matrix polymer or organic crystal particles is dispersed in a granular form. In forming a reversible thermal recording layer from such paint, the organic crystal particles are aggregated in the reversible thermal recording layer, and the level of dispersion of the organic crystal particles tends to be poor.
For example, in forming a reversible thermal recording layer from a water-based emulsion paint, the film forming process (a to c) in FIG. 8 takes place. More specifically, as water evaporates from the state of FIG. 8a in which a lot of water is contained in the coating layer, the matrix polymer is filled up with emulsion particles 7 (FIG. 8b). At this time, organic crystal particles 3 aggregate, and by directly heating the coating layer above the minimum temperature required for forming a continuous film (minimum film-forming temperature), a continuous film is formed as emulsion particles 7 fuse with each other. As a result, a reversible thermal recording layer in which organic crystal particles are poorly dispersed is formed.
When the organic crystal particles are poorly dispersed in the reversible thermal recording layer, the interface area of the organic crystal particles and matrix polymer decreases. The rate of organic crystal particles contributing to the opaque state is also lowered, thus lowering the transparent/opaque contrast and visibility.